285/75R16 vs 285/70R18
New overall diameter
33.71″
was 32.83″
True speed at an indicated 60
61.6 mph
speedometer reads low
Revolutions per mile
598
was 614 · −16
Switching from 285/75R16 to 285/70R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 285/75R16 | 285/70R18 | Change |
|---|---|---|---|
| Overall diameter | 32.83″ | 33.71″ | +0.88″ (+2.7%) |
| Sidewall height | 8.42″ (213.8 mm) | 7.85″ (199.5 mm) | −14.3 mm |
| Tread width | 285 mm | 285 mm | +0 mm |
| Circumference | 103.14″ | 105.90″ | +2.76″ |
| Revs per mile | 614 | 598 | −16 |
| Speedometer error | baseline | +2.7% | reads low |
Diameter overlay
285/75R16 285/70R18
Speedometer check
With 285/70R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.7 mph actual
35 mph shown
35.9 mph actual
45 mph shown
46.2 mph actual
55 mph shown
56.5 mph actual
65 mph shown
66.7 mph actual
75 mph shown
77.0 mph actual
Common questions
- Will 285/70R18 fit instead of 285/75R16?
- Good fit. Within the 3% rule — overall diameter changes only 2.7%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed. Overall diameter goes from 32.83″ to 33.71″, a change of +2.7%. As a rule of thumb, under 3% is safe, 3–5% needs a clearance check, and over 5% is not recommended.
- How far off will my speedometer read on 285/70R18?
- At an indicated 60 mph you would actually be travelling about 61.6 mph — a +2.7% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 285/75R16 and 285/70R18?
- 285/75R16 turns roughly 614 revolutions per mile; 285/70R18 turns roughly 598. That is a change of −16 revolutions per mile, which is what shifts odometer and speedometer readings.